DISPARATE PATHS IN THE GEOLOGIC EVOLUTION OF THE NORTHERN AND SOUTHERN APPALACHIANS; A CASE FOR INHERITED CONTRASTING LITHOSPHERIC SUBSTRATES

Modern understanding of the tectonic evolution of the Appalachians allows for recognition of most of the first-order lithotectonic elements and events of the orogen. Comparison of these features and events along the length of the orogen indicates that the northern and southern segments of the Appalachians display distinct first-order differences.

Contrasts between these segments of the orogen existed from the outset of the Appalachian cycle. Mesoproterozoic basement rock types south of Pennsylvania are different than those to the north and basement rocks in each area display distinct Nd and Pb isotopic signatures. A subsequent phase of c. 770-680 Ma, Cryogenian rifting is recorded in the southern Appalachians, but is missing in the northern part of the orogen. During the Paleozoic, the accretion of peri-Gondwanan terranes was partitioned, with Carolinia and Suwanee confined to the southern Appalachians and Ganderia, Avalonia, and Meguma limited to the northern Appalachians. Consequential to this partitioning, associated magmatism and some attendant tectonism is asymmetrically distributed between the two segments of the orogen. The terminal Appalachian collisional event, the Carboniferous Alleghanian orogeny, is distinctly different in the two segments of the orogen; there is an asymmetry with respect to the volume of magmatic rocks in the northern and southern Appalachians and their Carboniferous tectonic styles contrast sharply. Lastly, there is a modern first-order topographic change in the foreland of the orogen; the southern foreland is characterized by a continuous elevated plateau, whereas north of the New York promontory, foreland topography is more varied.

All of these varied first-order changes occur in the vicinity of the New York promontory, suggesting that it represents an enduring, fundamental boundary in the orogen. The nature and duration of differences between the two segments of the Appalachians indicate that this boundary was not an extrinsic ephemeral feature, such as a plate triple junction or hot spot. Rather, we suggest that an intrinsic difference in the Laurentian lithospheric substrate present from the outset of the Appalachian cycle, as reflected by contrasts in the Mesoproterozoic basement in each segment, could be the root cause of these significant contrasts.